1. Field of the Invention
The present invention relates to dynamoelectric machines, and more particularly to dynamoelectric machine rotors with superconducting windings. It can be used most advantageously in high-rated cryogenic turbine generators.
2. Description of the Prior Art
To protect a rotor superconducting winding from high electromagnetic fields caused by stator currents under abnormal conditions, the rotor winding is enclosed by an electromagnetic shield. The electromagnetic field produced by currents induced in the shield cancels out said external fields of the stator winding, thus preventing them from penetrating into the rotor winding. Interaction between the shield and stator fields is accompanied by the development of high dynamic forces acting on the shield, such as a dynamic torsional moment.
Known in the prior art is a dynamoelectric machine rotor with a superconducting winding (GB, A, 1351601) wherein the electromagnetic shield is provided with an outer casing made of titanium rings shrink fitted thereon to increase its strength. The shield is bolted to rotor and extensions supported in bearings, the shield and end extension being kept from rotation relative to a superconducting winding supporting structure by means of cotter pins.
The disadvantage of such a rotor lies in that a dynamic torsional moment arising under abnormal conditions, such as on shorting of the stator winding or nonsynchronous switching of the dynamoelectric machine into a network is completely transmitted to the shafting of the dynamoelectric machine due to a rigid connection between the electromagnetic shield and the supporting structure in a tangential direction. Torsional vibrations arising in the shafting produce therein substantial mechanical stresses which may result in its failure. Moreover, under abnormal conditions the electromagnetic shield is also subjected to radial forces bending and compressing the shield. These forces produce in the shield appreciable mechanical stresses, the outer casing having only a negligible effect on their values. At the same time, materials used for the electromagnetic shield, such as copper, have a high electrical conductivity required to provide efficient shielding, but as a rule do not possess sufficient mechanical strength to take up electrodynamic forces bending the shield of a high-rated turbine generator.
Also known in the prior art is a dynamoelectric machine rotor with a superconducting winding (Electrical Review International, v. 204, No. 14, 1979, April, England, "Commerical superconducting generator", p. 80-83), comprising an inner shaft formed by a superconducting winding supporting structure, an outer shaft formed by a cylindric shell encapsulating the rotor and rigidly fixed to end extensions at the ends thereof, and an electromagnetic shield including an inner cylinder made of material with a high electrical conductivity and an outer cylinder made of material with a high mechanical strength. The electromagnetic shield is fitted on the outer shaft with a radial interference.
In this rotor a maximum magnitude of a torsional moment transmitted from the shield to the shafting is defined by frictional forces between the shield and the shell and, hence, is dependent on the interference of their connection. However, compressive forces acting on the shield of a high-rated cryogenic turbine generator under abnormal conditions are rather high and may result in a substantial increase in frictional forces between the shield and the shell. Hence, a maximum moment transmitted to the shafting is also increased. Besides, when the shield is subjected to a high torsional moments, it may slip relative to the outer shaft. This slippage may be accompanied by injuries to the mating surfaces of the shield and outer shaft, appearing as scores, microwelds, ect., particularly when the outer shield is made of titanium, frictional forces increasing along with the increase of these injuries. Hence, a maximum magnitude of a tortional moment transmitted to the outer shaft is not only high, but will increase in the course of operation of a dynamoelectric machine. Thus, for a given rotor, with a high power of a dynamoelectric machine, there is also a danger of shafting failure under abnormal conditions.